Play slow, don't conquer too fast.
That is probably the best piece of advice that I could give you, too. Work your way slowly up to 45 provinces by 172 BC and leave yourself about 20 years to gradually change all of your provinces to Type II Governments. I personally stretched myself too thin going for 90 provinces and now my legions are dead and the Germans are systematically wiping out my mercenary armies. If you take it slow and develop your empire than you will find that you will be making more money, have secure borders, and you won't need to manage as many territiories. By the time the Marians take effect your Imperium will be the greatest (maybe not the largest) in the world, and you can unleash your legions on the barbarians. (kill some Sweboz for me ;)